The present invention relates to a nuclear magnetic resonance spectroscopy method and to a nuclear magnetic resonance spectrometer adapted to detect selectively the existence of weak, or long-range, couplings between carbon and hydrogen nuclei.
Determination of the number of hydrogen atoms coupled to each carbon from a carbon-13 NMR spectrum and subsequent elucidation of spin coupling between the hydrogen and carbon nuclei are indispensible for analysis of the structure of organic compounds. In carbon-13 NMR spectra where the carbons are left coupled to the hydrogen nuclei, each carbon resonance splits into a multiplet due to the spin coupling with hydrogen nuclei. Spin couplings of this kind are classified as direct spin coupling that is produced between directly bonded carbon and hydrogen and weak, or long-range, coupling that is observed between carbon and hydrogens which are separated by more than two or three chemical bonds.
Methods which are now available to elucidate such a long-range coupling are (1) long-range selective proton decoupling (LSPD) spectroscopy and (2) long-range C-H coupling resolved two-dimensional spectroscopy (resolution). In the long-range selective proton decoupling, a hydrogen nucleus which corresponds to a certain peak appearing in a proton NMR spectrum is decoupled by radio-frequency irradiation. Under this condition, a carbon-13 NMR spectrum is obtained, and this is compared with another spectrum obtained under non-decoupled condition to examine the difference. The LSPD is the fundamental method for interpreting carbon resonance lines.
The LSPD causes each resonance line to be split into a very complex multiplet pattern when a single carbon is spin-coupled to too many hydrogens, as often encountered in saturated compounds. Under this condition, even if the hydrogen at one location is decoupled, the change produced thereby can be hardly observed.
The above-mentioned drawback with the LSPD can be solved by the use of the long-range C-H coupling resolution which is a kind of two-dimensional NMR and able to resolve resonance lines attributed to weak, long-range couplings into simpler and clearer splitting patterns. This long-range C-H resolution is described in Tetrahedron Letters, Vol. 25, No. 3, pp. 337-340, 1984. However, this method has various disadvantages. First, it requires very long time for measurement. A reduction in the signal-to-noise ratio necessarily results due to the presentation of power spectrum using special window functions. A memory having a large storage capacity is needed. Further, a computer program for two-dimensional NMR is necessitated. Thus, the long-range C-H resolution is not easy to perform.